Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices such as, for example, a urea-based Selective Catalytic Reduction (SCR) catalyst, which is capable of continuously reducing NOx emissions, even in an oxygen rich environment. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream. The heat in the exhaust gas causes the aqueous urea solution to decompose into ammonia and hydro-cyanic acid (HNCO). These decomposition products enter the SCR where the gas phase ammonia is adsorbed and the cyanic acid is further decomposed on the SCR to gas phase ammonia. The adsorbed ammonia then takes part in the reduction of gas phase NOx.
The inventors herein have recognized a disadvantage with this approach. Namely, optimum system efficiency is not achieved unless the injected aqueous urea is well distributed and atomized and has sufficient time in the exhaust gas to complete conversion into gaseous ammonia prior to reaching SCR catalyst brick face. The inventors have further recognized that a typical prior art urea injection system is not effective in achieving this because it sends out a directed spray of urea that does not mix well with the exhaust gasses. Further, this spray of urea might impact the catalyst brick directly, thus depositing aqueous urea on it and degrading the catalyst.